1. Field of Invention
The present invention relates to an electrically controlled, pressured fluid actuated, three-position actuator and, in particular, relates to a solenoid controlled pneumatic actuator for selectively causing engagement of a selected one of three selectable auxiliary section ratio gears in a vehicular compound mechanical transmission.
2. Description of the Prior Art
Compound mechanical transmissions having three-speed auxiliary sections, usually splitter type auxiliary sections, connected in series with a multiple speed main transmission section are well known in the prior art. An example of this type of transmission is sold by Eaton Corporation and its wholly-owned subsidiary, Eaton Limited, under the trademark "TwinSplitter" and is illustrated in U.S. Pat. No. 4,736,643, the disclosure of which is incorporated herein by reference.
Shifting actuators for such transmissions typically comprise a first shift fork controlled by a first, three-position piston having a centered nonengagement position and selectably displaceable in both axle directions to two selectable engagement positions and a second shift fork controlled by a second, two-position piston having a nondisplaced nonengagement position and selectably displaced to a single engaged position. An interlock assembly is provided to assure that only one shift fork is displaced from its nonengaged position at any given time. Actuator piston assemblies of this type may be seen by reference to U.S. Pat. No. 4,748,863, the disclosure of which is incorporated herein by reference.
The prior art devices, while providing their intended functions, were not totally satisfactory as they usually included differential area pistons which, for a given size of actuator, reduces the force available for some shifts which may be especially objectionable during cold start up conditions and usually included and air control system which would default to auxiliary section section neutral in the event of a solenoid and/or pneumatic power failure.